AQUATIC MAMMALS 



pubis has probably been eliminated and the ischium shortened and con- 

 torted. This situation is broadly similar to what is indicated in a zeu- 

 glodont, as discussed shortly, but very different indeed from what has 

 taken place in the dugong. Clearly the reduction in the hind limb of 

 these two families of sirenians has been upon entirely different prin- 

 ciples, likely involving fundamental difference in the uses to which the 

 hind limbs were previously put, and it seems unsafe to hazard any opin- 

 ion in regard to just what particulars of the disappearing hind limbs 

 may have been involved. 



In other respects the modications of the Sirenia are much more 

 puzzling than in the Cetacea and it is correspondingly difficult to recon- 

 struct the probable steps that were followed. In the case of the whales 

 the salient characteristics are such as to point clearly, to me at least, to 

 numerous hypotheses that have a high degree of probability. This is 

 not the case m the Sirenia. It is difficult for me to envision any sort of 

 primitive mammal of proboscidean affinity taking to the water in the 

 sluggish manner which one would naturally connect with the sirenian 

 prototype and developing the tail which we now see in connection with 

 the total disappearance of the hind limbs. Evidently the latter situa- 

 tion followed abandonment of any use of these members, but more I do 

 not feel like saying at the present time, for further statement would be 

 pure, unsupported speculation. 



Indubitably many groups of the Cetacea have been distinct from one 

 another for a great length of geologic time, presumably since the period 

 when they had external hind limbs which were quite well formed. As 

 the latter tended toward more and more complete disappearance there 

 naturally occurred differences in precise details. Hence it is entirely ac- 

 cording to expectation that in examining the anatomy of this order one 

 finds hardly any two distinct sorts with the details of the pelvic muscu- 

 lature entirely the same. In fact the differences are so great that it is 

 often difficult to correlate the pelvic muscles of two sorts of whales, and 

 utterly impossible yet to homologize them with muscles of terrestrial 

 mammals for the reason that the nerves are so greatly altered, and in the 

 present state of our knowledge it is unwise even to attempt to do so. 



The cetacean innominate is now so different from the generalized con- 

 dition that very little can be proven about the homology of its parts, nor 

 does the fossil material, except in the Archaeoceti, help us to any ex- 

 tent. Usually one area of the bone is thicker and broader than the rest, 

 and this, presumably, marks the previous situation of the hip joint, but 

 there may be exceptions. In some cases existing projections from this 



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